flush pending aio requests

When we finish migration, there may be pending async io requests
in flight. If we don't flush it before stage3 starting, it might be
the case that the guest loses it.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

migration.c

@@ -266,6 +266,7 @@ void migrate_fd_put_ready(void *opaque)
         dprintf("done iterating\n");
         vm_stop(0);
 
+        qemu_aio_flush();
         bdrv_flush_all();
         if ((qemu_savevm_state_complete(s->file)) < 0) {
             if (old_vm_running) {